During the past several years, there has been a significant increase in the use of the one visit procedure (N2) for the treatment of endodontic patients by dental practitioners in the United States. Based on scientific evidence, the N2 method is not taught in any dental school in the U.S. However, the evidence available is difficult to interpret since there is an obvious absence in the literature of studies that directly compare the N2 method with the method accepted in the United States. The study proposed here will evaluate each method by a direct comparison of three critical aspects of treatment. These include: canal preparation and debridement; canal obliteration; and the reaction of infected periapical tissues to each method of treatment. Accepted in vitro procedures will be utilized to compare canal preparation and obliteration. Clinical treatment of pulpally and periapically infected monkey teeth, using a standardized infecting procedure, will be evaluated histopathologically on the basis of periapical tissue vitality and repair; arrangement of the periodontal ligament at the apical foramen and the presence of residual bacteria. Results of these studies will give more accurate means for evaluating each treatment method.